This trial is designed to evaluate whether four drug combination therapies based on prior antiretroviral experience can maximally decrease viral load and increase CD4 cells. The primary objectives are 1) to determine the proportion of subjects in each Group having reduction in HIV-1 RNA when all antiretroviral agents are changed and at least 2 of the agents are novel and to determine the duration of the reduction; 2) To evaluate the safety and tolerance of these combinations based on clinical and laboratory toxicities; and 3) to determine the steady-state pharmacokinetics of nevirapine, nelfinavir and ritonavir when used in combination with respect to age and treatment group.